Estimated reading time: 3 minutes

Trouble in Your Tank: The Role of Organic Solderability Preservatives in Advanced Packaging

Technology trends shaping the electronics industry supply chain—AI, IoT, ADAS, and high-performance computing (HPC)—are driving finer circuit features and higher layer counts. Advanced packaging drives the selection of surface finishes depending on the application.

Typical designs require excellent solder joint reliability and wire bondability. It is not uncommon for designers to specify the organic solderability preservatives (OSP) on the BGA side of the substrate and precious-metal-plated finishes on the top side, which facilitate wire bonding. This is essentially a process of selective surface finishing. The OSP is particularly useful for fine-pitch BGAs with pitches below 1.0 mm. In addition, the solder joint formed with an OSP finish is quite strong, as the tin in the solder forms a direct intermetallic bond with the copper.

The Trend Toward Mixed Metal Finishes
To achieve high-density surface mounting on printed wiring boards (PWBs), the number of terminals on circuit components has been increasing, and the pitch has been significantly reduced. With the trend toward increased packaging density has come the use of chip-on-board (COB), flip chip, and tape automated bonding (TAB).

In many instances, the surface mounting of such components may be required on PWBs with copper pads and other features plated with gold, silver, tin, or solder. These mixed-metal finish boards are becoming increasingly common, and the surface treatment of such circuits is becoming more important. The demand was such that a water-soluble surface-treating agent capable of protecting bare copper from oxidation without leaving a film on other metals needed to be developed and implemented. In other words, the need for an OSP that selectively bonds to the copper without adversely affecting other metals, such as gold or solder, was established (Figure 1).

Conventional OSP processes, based on long-chain alkylimidazole compounds and substituted benzimidazole compounds, have functioned adequately to protect the bare copper. However, these materials also deposited a significant film on other metals such as gold, tin, and solder. This additional film interfered with subsequent operations, such as wire bonding and surface mounting of quad flat packs on solder surfaces.

In addition, the contact resistance on the gold increased to unacceptable levels. Thus, when PWBs are fabricated with multiple metal finishes, the metals (such as gold or solder) must be masked to prevent OSP film formation on their surfaces. In some instances, the coating must be removed with alcohol, adding additional labor and cost to the fabrication process.

One factor in promoting this film formation on the metal surfaces is the copper present in many organic solderability formulations. The copper ions form a complex with the active azole ingredient in the OSP chemistry and actually help to promote film growth. When a copper-solder mixed-metal board is processed through such a process, the OSP forms on the solder and discolors it, making long-term solderability virtually impossible to achieve.

It has also been determined that the copper ions in the OSP protective film contribute to ionic contamination, a situation that is constantly scrutinized by assembly houses and end users. It is desired to keep ionic residues as low as possible. It has been demonstrated that the copper contributes to the staining/darkening of the solder and causes undue build-up of residue on the gold.

The Solution
The optimal solution to prevent OSP deposition on gold and improve solderability under lead-free assembly conditions is based on a unique organic compound: an imidazole synthesized as the active ingredient in the OSP (US Patent 5,795,409). This unique compound is solubilized in water and a nominal amount of acetic acid. Acetic acid helps maintain a buffered pH in the OSP process.

This process employs a combination of a phenyl imidazole compound mixed in an aqueous solution with acetic acid, a complexing agent, and a water-soluble iron compound. The combination of these additives allows the uniform coating of the organic film on the copper without building any appreciable amount on the non-copper surfaces. After the PWB has been prepped in an acid cleaner, followed by a micro-etch, the OSP coating is applied to the PWB by dipping, spraying, or flood coating.

Figure 2 illustrates the latest generation OSP active compound. The substituted phenyl imidazole represents a significant improvement in solderability protection as well as preventing subsequent OSP film deposition on other metals such as gold.

Therefore, it was imperative to develop an OSP process that would selectively deposit on the bare copper surfaces only, with low residual ionics. However, a film that forms on the copper must have sufficient ability to maintain the solderability of the base copper through multiple thermal excursions and with a variety of low-activity wave soldering fluxes and pastes.

The efficacy of this process is demonstrated in Figure 3. Note the discolored gold deposits on the left with the benzimidazole compound. The as-plated coupon is shown in the middle, and the gold-plated coupon processed through the improved OSP is shown on the right.

We should not underestimate the importance of minimizing organic deposits on gold, as they can interfere with successful wire bonding.

In a future column, we will take a more detailed look at OSP technology for advanced packaging.

References

“Understanding the Basics of the Wire Bonding Process in Semiconductor Packaging,” by Ross Feng, Viasion.

This column originally appeared in the May 2026 issue of I-Connect007 Magazine.